Surface modification of pyrolyzed carbon fibres by cyclic voltammetry and their characterization with XPS and dye adsorption

Electrochimica Acta 55 (2010) 1207–1216

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Surface modification of pyrolyzed carbon fibres by cyclic voltammetry and their characterization with XPS and dye adsorption P. Georgiou a , J. Walton b , J. Simitzis a,∗ a National Technical University of Athens, School of Chemical Engineering, Department III, “Materials Science and Engineering”, Laboratory Unit “Advanced and Composite Materials”, 9 Heroon Polytechiou Str., Zografou Campus, 157 73 Athens, Greece b Corrosion and Protection Centre, School of Materials, The University of Manchester, P.O. Box 88, Manchester M60 1QD, UK

a r t i c l e

i n f o

Article history: Received 21 July 2009 Received in revised form 24 September 2009 Accepted 25 September 2009 Available online 7 October 2009 Keywords: Carbon fibre Pyrolysis Cyclic voltammetry XPS Dye adsorption

a b s t r a c t Commercial carbon fibres were pyrolyzed up to 1000 ◦ C and were then electrochemically treated by cyclic voltammetry in aqueous electrolyte solutions of H2 SO4 , in two potential sweep ranges: a narrow region, N, and a wide region, W, avoiding and including water decomposition, respectively. The anodic and cathodic peaks were correlated with oxide formation and their partial reduction, respectively. The nature of oxygen containing groups on the fibre surfaces was determined by XPS. Wide scan spectra and high energy resolution spectra were recorded through the C 1s, O 1s, N 1s and S 2p photoelectron regions. The ability of the fibres to adsorb methylene blue and alizarin yellow dyes from their aqueous solutions indicates the presence of electron acceptor or donor groups on the fibres, respectively. The carbon fibres were classified into two categories. The first includes electrochemically untreated and treated in the N region, and the second those treated in the W region. The high oxygen concentration and effective dye adsorption on the carbon fibres in the second category indicates that their surfaces were effectively modified. The adsorption of dyes on carbon fibres constitutes a complementary method to XPS for an indirect estimation of oxygen and other groups present on the carbon fibre surfaces. © 2009 Elsevier Ltd. All rights reserved.

1. Introduction Carbon fibres are mainly produced using textile fibres based on polyacrylonitrile (PAN) as precursors. During their production desirable properties, such as high modulus or high strength can be controlled [1]. Carbon fibres find important applications in reinforcing composite materials due to their excellent mechanical strength, high heat-conductivity, light weight and hydrophobicity, in combination with their acceptable price [2,3]. Interfacial interactions between carbon fibres and the matrix play a crucial role in the performance of the fibre reinforced composites [4,5] as exemplified by failures due to debonding of the fibres from the matrix as a result of weak adhesion, and due to the formation of a brittle composite as a result of a strong interaction between the fibres and the matrix [4]. The inert nature of carbon fibre surfaces may be modified by oxidation. The processes to achieve this can be categorized into chemical “wet” methods using acids and salts, and chemical “dry” methods such as oxidation in air, oxygen, ozone, plasma treatments, heating, radio-frequency glow discharge, and microwave discharge. They also include electrochemical meth-

∗ Corresponding author. Tel.: +30 210 7723178; fax: +30 210 7723252. E-mail addresses: [email protected], [email protected] (J. Simitzis). 0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2009.09.068

ods such as anodic oxidation in various electrolytes, and coating methods such as electro-polymerization, pyrolytic graphite, and metal oxide coating [1,3,6]. These treatments lead to the formation of functional groups on the carbon fibre surface and to an increase in the roughness of their surface. Both improve fibre adherence to the matrix either by creating chemical bonds, van der Waals interactions, bipolar interactions, or by providing mechanical interlocking [3,5,7,8]. Furthermore, extensive electrochemical oxidation produces porous carbon fibres with adsorbent capabilities [9]. One of the electrochemical methods used to analyse and modify fibre surfaces is Cyclic Voltammetry (CV), which enables controllable treatment conditions [9,10]. CV is an electroanalytical method for simultaneous activation of molecules by electron transfer and study of the corresponding chemical reactions. Specifically, the CV response curve provides useful information relating to the reaction mechanism, the electron transfer kinetics and thermodynamics, the effects of electron transfer e.g. functional groups formation, and furthermore it enables identification of side and final products of electrochemical reactions [11,12]. The anodic peaks are attributed to oxidation phenomena and the cathodic peaks to reduction phenomena on the carbon fibre surface. The minimum potential for oxygen evolution depends on the temperature, the pH, the type and concentration of electrolyte and the type of electrode [13]. Aqueous solutions of weak or semi-concentrated sulfuric acid have

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Table 1 Conditions of cyclic voltammetry used for the surface modification of pyrolyzed carbon fibres. Sample

P P-C20-N-20ca P-C5-W-20c P-C20-W-20c P-C50-W-20c

Untreated pyrolyzed carbon fibres (symbol: P)

X X X X X

Conditions of electrochemically treated pyrolyzed carbon fibres H2 SO4 concentration (%, w/w) (symbol: C)

Scanning sweep range vs SCE(V) (symbol: N or W)

Scanning sweep rate (mV/s)

No. of cycles (symbol: c)

Total time (min)

– 20 5 20 50

– −1 → +1.5 → −1 (N) −3 → +3 → −3 (W) −3 → +3 → −3 (W) −3 → +3 → −3 (W)

– 50 50 50 50

– 20 20 20 20

– 33.33 80 80 80

a The corresponding materials for 1st, 2nd, 5th, 10th, 15th and 20th cycles have the codes: P-C20-N-1c, P-C20-N-2c, P-C20-N-5c, P-C20-N-10c, P-C20-N-15c, P-C20-N-20c, respectively.

a “decomposition potential” in the vicinity of +1.7 V on a smooth platinum electrode, resulting in the formation of nascent (atomic) oxygen [8,13]. X-ray photoelectron spectroscopy (XPS) is a powerful technique for the investigation of carbonaceous surfaces due to its ability to provide chemical state information from the top few atomic layers of a surface [14,15]. The surface oxidation of carbon fibres is observed as a shift to higher binding energy from the graphitic peak [1,16]. Dye adsorption can also be used for the characterization of the functional groups of carbonaceous material (adsorbent) based on the “electron donor–acceptor interactions” between its groups and those of the dye (adsorbate) [17–22]. In this work the commercially available carbon fibres with unknown history were studied. These carbon fibres will be pyrolyzed at 1000 ◦ C in order to destroy oxygen groups at the surface prior to their modification by cyclic voltammetry. The formation of functional groups during their modification will be correlated with the groups that will be determined with XPS analysis and dye adsorption.

2. Experimental 2.1. Electrochemical treatment of carbon fibres Prior to electrochemical treatment, commercial PAN based high tensile carbon fibres, cut to approximately 7 cm, were placed on a ceramic specimen carrier and pyrolyzed in a nitrogen atmosphere (99.999%), by heating to 1000 ◦ C, at a rate of 10 ◦ C/min, remaining at this temperature for 30 min. The pyrolyzed carbon fibres were surface modified by CV using a Potentioscan POS 88, (Bank Electronics), connected to a three-compartment glass electrochemical cell, and the data recorded on a personal computer. The three electrodes of the electrochemical cell consisted of a platinized plate as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode and the pyrolyzed carbon fibres (single bundle) as the working electrode. CVs were acquired over both narrow and wide potential ranges. In the narrow potential range, N region, the voltage was swept from −1.0 to +1.5 V and back to −1.0 V, in a 20% (w/w) aqueous solution of H2 SO4 , avoiding water decomposition. In the wide potential range, W region, the voltage was swept from −3.0 to +3.0 V, and back to −3.0 V, in 5%, 20% and 50% (w/w) aqueous solutions of H2 SO4 , leading to water decomposition with evolution of oxygen and hydrogen. The CVs were started at cathodic potentials for the reduction of possible contaminants already present on the carbon fibre surfaces. The electrochemical treatment was performed at room temperature for 1–20 cycles, in both potential ranges, at a rate of 50 mV/s. Additionally, carbon fibres were electrochemically treated in 95–97% (w/w) aqueous solution of H2 SO4, from −1.0 to +2.0 V and back to −1.0 V, for 5 cycles, at a rate of 2.5 mV/s.

2.2. X-ray photoelectron spectroscopy XPS was undertaken on a Kratos Axis Ultra photoelectron spectrometer located in the School of Materials at the University of Manchester. The fibres were mounted laterally across a sample bar, attached to the side rails of the bar using strips of adhesive tape. In this way, there was no supporting material beneath the analysis area. Charge build up at the sample surface was minimised using a low-energy electron flood source. Wide scan spectra were recorded for elemental quantification, and high energy resolution spectra for chemical state determination were recorded through the C 1s, O 1s, N 1s and S 2p photoelectron regions [23,24]. Curve fitting was carried out on the high energy resolution spectra from the C 1s and O 1s photoelectron regions to resolve overlapping peaks, and the data was corrected for charging effects by reference to the graphitic carbon peak at 284.6 eV binding energy [2,9,25–29]. All data processing was performed using CasaXPS version 2.2.101 [30]. 2.3. Dye adsorption Carbon fibres were added in a dye aqueous solution of methylene blue or alizarin yellow (0.032 g/L) with a ratio of 1.65 g/L. The colour of the solution was determined optically at specific times by comparison with a colourimetrical calibration scale and after reaching the equilibrium by UV–visible spectrophotometry at 665 nm for methylene blue and at 375 nm for alizarin yellow. The UV–visible measurements were performed on a Cary 300 Conc Varian UV–visible Spectrophotometer using the Cary Win UV Scan Application Software (version 02.00 (25) EPROM version 9.00). 3. Results and discussion 3.1. Electrochemical treatment of carbon fibres Table 1 presents the conditions for the electrochemical treatment of pyrolyzed carbon fibres, in (a) the narrow, N region, −1.0 to +1.5 V and back to −1.0 V and (b) the wide, W region, −3.0 to +3.0 V and back to −3.0 V. Fig. 1 shows the cyclic voltammograms of the carbon fibres treated in 96% (w/w) H2 SO4 in the region −1.0 to +2.0 V and back to −1.0 V for the first to the fifth cycle, at a scanning rate of 2.5 mV/s. Some theoretical aspects should firstly be mentioned in order to interpret the anodic and cathodic peaks of voltammograms. Electrochemical intercalation reactions are confined to electron/ion transfer reactions. During the cathodic reduction or the anodic oxidation of carbon electrodes, guest ions, cations M+ or anions X− , penetrate into the van der Waals gaps between the carbon layers and enlarge the inter-layer distance [31]. Cn + X− − e−

anodic oxidation



Cn+ X−

cathodic reduction

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ture the term of GO typically indicates chemically formed graphite oxides, however very often there is no difference between GO and EGO, which is also valid in this paper. The GO are bulk oxides that have their oxygen functionalities between the carbon planes. On the other hand, the surface oxides, mainly –COOH and ♥COH groups [33] are only found at plane edges, which are more reactive, or in lattice defects on the basal planes, in contrast to the basal planes themselves that expose the relatively stable ␲-electronic system [31,32]. Extensive oxidation of graphite removes electrons from the aromatic ␲ system, resulting in the formation of aliphatic bonds in the graphite plane [38]. The cathodic peaks c1 and c2 result from reduction reactions and correspond to:

Fig. 1. Cyclic voltammograms of the carbon fibres treated in 96% (w/w) H2 SO4 in the region (−1 V → +2 V → −1 V) for first up to 5th cycle, with scanning rate of 2.5 mV/s.

Strong acids with sufficient oxidizing power such as H2 SO4 and HNO3 can be used directly for intercalating the graphite. In these cases, the acids serve as intercalating oxidants as well as solvents. Since excess acid is always present during intercalation, the acid molecules are also trapped with intercalant anions to form compounds such as Cx + HSO4 − (H2 SO4 )y [32]. Concerning Fig. 1, inspecting the first cycle, beginning from −1.0 V, four anodic peaks, a1 –a4 are observed up to +2.0 V and two cathodic peaks c1 and c2 from + 2.0 up to −1.0 V. The anodic peaks a1 –a4 result from oxidation reactions and correspond to: a1 at approx. +0.7 V is a pre-peak due to intercalation into a decomposed graphite intercalated compound (GIC), i.e. a previously intercalated and hence “unlocked” graphitic structures [33]. For the pyrolyzed fibres used in the present work this intercalation could take place in such graphitic structures that are decomposed during the pyrolysis. a2 at approx. +1.0 V corresponds to the formation of lamellar (=planar carbon layer) sp2 -type graphite salts [34,35] and especially to the formation of stage II, C48 + HSO4 − ·2H2 SO4 , i.e. sulfuric acid GIC, indicating intercalated anion per two graphene layers [31]. The ideal composition includes two molecules of sulfuric acid, which molecules can occupy positions of anions in the case of insufficient oxidation [31,33,34]. a3 at approx. +1.2 V corresponds to the formation of stage I, C24 + HSO4 − ·2H2 SO4 , i.e. sulfuric acid GIC, indicating intercalated anion per one graphene layer [31,35]. Furthermore, the anodic peak a3 can also be attributed to the formation of ‘defect oxides’ derived from defect positions of graphitic structure [35]. a4 at approx. +1.5 V corresponds to further oxidation to partly covalent graphite oxide ‘GO’ [35,36]. There may be three steps in the electrochemical formation of a graphite oxide: (i) formation of a stage II or I bisulfate GIC [37] (see peaks a2 and a3 , respectively) (ii) electro-oxidation of the formed GIC by the further charging and (iii) hydrolysis of the formed GIC and loss of sulfuric molecules [37] (for steps (ii) and (iii) see peak a4 ). Concerning the graphite oxide ‘GO’, there is a difference between the surface oxides and the GO or EGO (electrochemically graphite oxides). In the litera-

c1 at approx. +0.5 V corresponds to reduction of graphite oxides ‘GO’ and to de-intercalation reactions [35]. When the intercalation compound of graphite is reduced, the original graphite is formed via reversible redox processes. When graphite intercalation compounds of H2 SO4 are further oxidized by using a higher anodic potential in concentrated H2 SO4 solution, the oxidation processes are irreversible and the reduced product is quite different from the original graphite [38]. Electrochemically, the ‘GO’ have very different reduction behaviour to that of the ‘surface oxides’. For the ‘GO’, the potential distance between the anodic oxidation peak and the cathodic reduction peak is very close, unlike the ‘surface oxides’ (except for some types of quinonoid groups) [31]. Indeed, based on the voltammogram, the Epa4 − Epc1 is equal to 1.0 V for ‘GO’ (bulk oxides) and Epa4 − Epc2 is equal to 2.0 V for ‘surface oxides’. Oxide formation in the bulk of the GIC system, during the oxidation peak a4 , can cause irreversible damage to the graphite lattice and therefore irreversible de-intercalation [32] occurs during the peak c1 . The de-intercalation reactions correspond to this potential, but they are overlapped by the reduction peak of graphite oxides ‘GO’ [31,35]. c2 at approx. −0.5 V corresponds to the reduction of ‘surface oxides’, ‘defect oxides’ and ‘partially reduced GO’ [31,39]. ‘Defect oxides’ arise during the graphite oxide ‘GO’ formation, anodic peak a4 , as a side reaction, without restoring, i.e. damaging, the carbon layers and leaving activated C sites [35]. During the electrochemical treatment in the region of peak a4 , anodic ‘bulk’ oxidation of graphite to partly covalent graphite oxide ‘GO’ occurs, which during the subsequent cathodic reduction leaves carbon with a large fraction, of the order of 5–15%, of “sp3 sites” in the original carbon layers [36]. A corresponding fraction of covalently bonded residual groups which may serve as “anchor groups” [31] remain after incomplete cathodic reduction of ‘GO’. Even at the potential of the hydrogen electrode (≈−0.25 V vs. SCE) these residual groups (‘residual GO’) are not yet reduced to carbon, since complete reduction requires approximately −0.75 vs. SCE) [36]. As ‘residual GO’, ‘rGO’ characterizes the ‘partially reduced GO’, which obtain many functional acidic groups with strong ion exchange properties [31]. Inspecting the second cycle, four anodic peaks, a1 –a4 are observed during the forward sweep and three cathodic peaks, c1 –c3 during the reverse sweep. The same behaviour as that of the first cycle is observed with the exception of the cathodic peak c3 , which is absent in the first cycle. The peak c3 is observed at approx. −0.9 V. This peak corresponds to reduction of O2 present in the electrolyte solution to superoxide anion radicals • O2 − that takes place in aprotic or protic medium. Although this intermediate product is the same for both cases, the final products are different [40–43]. This ion is fairly stable in aprotic solvents (e.g. in DMF its half-life time is t1/2 = 75 min); however it has a very short life in aqueous solutions [43,44].

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Fig. 2. Cyclic voltammograms of the carbon fibres treated with scanning rate of 50 mV/s in: (a) the N region (−1 V → +1.5 V → −1 V) in 20% w/w H2 SO4 for different cycles up to 20, (b) the W region (−3 V → +3 V → −3 V) in 5%, 20% and 50% (w/w) H2 SO4 for 20 cycles.

During the forward sweep from the second up to the fifth cycle, the anodic peaks a1 and a2 are observed at approximately the same potential. However, the anodic peak a3 is shifted to more positive potentials, indicating difficulty in the formation of stage I intercalation and simultaneously the anodic peak a4 is shifted to less positive potentials, favouring the formation of ‘GO’. After the fourth cycle the previous a3 and a4 give one broad peak. On increasing the number of cycles, the current density of the anodic peak a1 is reduced, and practically disappears by the fifth cycle. The corresponding current densities of the anodic peaks a2 and a3 increase from the first to the second cycle, and then they are reduced. During the reverse sweep from the second up to the fifth cycle, the cathodic peaks c1 and c2 are observed at approximately the same potential. The current densities of the cathodic peaks c1 , c2 and c3 reduce with increasing cycles and by the fifth cycle the peak c3 has disappeared. As previously discussed for peak c1 , the anodic and cathodic peaks of the voltammograms are irreversible, i.e. the oxides formed during the anodic treatment, from peaks a2 to a4 , are only partially reduced during the cathodic treatment, from peaks c1 to c2 , and therefore they remain on the carbon fibre surface. On increasing the number of cycles, the defect sites are reoxidized to functional groups, corresponding to peak a3 , and therefore more irreversible damage is done to the lattice during the cathodic reduction of oxides [35]. Fig. 2a shows the cyclic voltammograms from the pyrolyzed carbon fibres treated in the narrow, N region, and the peak characteristics based on the curves are listed in Table 2. During the forward sweep, from −1.0 to +1.5 V of the first cycle, one anodic peak is observed at approximately + 0.2 V due to the oxidation of adsorbed hydrogen (carbon–Hads − e− → carbon + H+ ) [45] or other gaseous species, which are evolved during pyrolysis [46,47]. Furthermore, this peak is attributed to adsorption of oxygen from solution onto the graphite surface during the anodic treatment, which is therefore available to oxidize the carbon present at the active sites [48] during the next cycles. From the second up to the twentieth cycle, the anodic peak is shifted to more positive poten-

tials, at approximately +0.4 up to +0.6 V, leading to the formation of ‘surface oxides’. The formation of quinone-like oxides occurs at +0.4 V [49–52]. Additionally, oxygen evolution on the carbon fibre electrode is not observed in this potential region during the anodic sweep. This is in agreement with the voltammograms in the literature [10,53] where oxygen evolution occurs at higher potentials, like those of Fig. 2b, above +1.8 V. Inspecting the reverse sweep, from +1.5 to −1.0 V, at the first cycle a cathodic peak is observed at approximately +0.24 V. This peak is attributed to the reduction of quinoid groups to give hydroquinone structures [52,54]. With increasing number of cycles, this cathodic peak is observed at approximately the same potential value. Furthermore, after the first cycle, an increase of the current density is observed from −0.4 to −0.5 V, indicating the reduction of ‘surface oxides’ and ‘defect oxides’ [39] which it was previously described as cathodic peak c2, in Fig. 1. Additionally, hydrogen evolution on the carbon electrode is not observed during the cathodic sweep in this potential region. This is in agreement with the voltammograms in the literature [10,45] where hydrogen evolution occurs at lower potentials, like those of Fig. 2b, i.e. below −1.25 V. Table 2 presents the quantitative characteristic data based on Fig. 2a and b. The anodic and cathodic peak potentials, Epa and Epc refer to the maximum point of the corresponding peak and Ep is their difference (Ep = Epa − Epc ). The peak current density, ip , (as anodic, ipa, or cathodic, ipc ) is determined from the voltammograms i = f(E) according to the literature [11,42], and then the −1 ratio of |ipa × ipc | is calculated. The charge Qp (Q = i × t) is determined from the graphs i = f(t), using the trapezoidal method [55], for the anodic (Qpa ) and cathodic peak area (Qpc ) and then the ratio −1 of |Qpa × Qpc | is calculated. The anodic and cathodic potential limits, p.l., indirectly indicate the oxygen or hydrogen evolution on the carbon fibre electrodes, respectively. The evolution of both gases is also observed during the experiment. The anodic and cathodic p.l. are determined in the linear region at the end of the anodic and of the cathodic peak, respectively, by extrapolating the slope to the horizontal axis, and the corresponding values at the intercept are presented in Table 2.

−1.69

– –

2.09

– –

−1.53

– –

1.98

– –

−1.25

– –

1.80

– –

1.55 11.72 0.85 1.28 up to 2.49

0.688

−0.812

18.13

1.20 32.61 0.83 1.86 up to 3.12

1.968

−2.359

39.06

0.97 80.11 0.77 3.411 2.43 up to 3.52

−4.405

77.58

3.42 2.45 8.39 2.17 0.268 0.582 0.38 up to 0.20

2.14 3.39 7.24 1.52 0.517 −0.05 up to 0.88

0.341

1.72 3.29 5.66 1.27 0.413 −0.11 up to 0.77

0.326

1.41 3.69 5.19 0.997 0.368 −0.17 up to 0.59

0.369

0.94 4.47 4.22 0.82 0.352 −0.34 up to 0.49

0.429

0.93 3.96 3.68 0.80 0.423 0.337

P-C50-W-20c

P-C20-W-20c

P-C5-W-20c Fig. 2 (b)

P-C20-N-20c

P-C20-N-15c

P-C20-N-10c

P-C20-N-5c

P-C20-N-2c

0.40 up to 0.07 (max. 0.24) 0.36 up to 0.02 (max. 0.19) 0.37 up to 0.03 (max. 0.22) 0.33 up to −0.04 (max. 0.11) 0.27 up to −0.08 (max. 0.16) 0.38 up to 0.00 (max. 0.19) −1.43 up to −1.77 (max. −1.63) −1.04 up to −1.40 (max. −1.25) −0.61 up to −0.87 (max. −0.71, −0.82) Fig. 2 (a)

P-C20-N-1c

−0.04 up to 0.37 (max. 0.16 up to 0.20) 0.02 up to 0.51 (max. 0.35 up to 0.41) 0.20 up to 0.62 (max. 0.48) 0.22 up to 0.73 (max. 0.54) 0.22 up to 0.80 (max. 0.57) 0.76 up to 0.20 (max. 0.56 up to 0.58) 1.00 up to 1.75 (max. 1.40) 0.82 up to 1.72 (max. 1.32 up to 1.47) 0.67 up to 1.62 (max. 1.50 up to 1.55)

−0.44 up to 0.30

Anodic (V) Anodic Qpa (mC cm−2 ) Anodic ipa (mA cm−2 ) Cathodic Epc (V)

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The reversibility or irreversibility of the redox reactions can be distinguished based on the anodic and cathodic curves [11]. For reversible reactions, the following two of the criteria should simultaneously hold: (i) Ep should satisfy the equation Ep =

Anodic Epa (V)

Peak potential Sample

Table 2 Peak characteristics determined from cyclic voltammograms.

Ep = Epa − Epc (V)

Maximum peak current density

Cathodic |ipc | (mA cm−2 )

Ratio of −1 |ipa × ipc |

Peak charge

Cathodic |Qpc | (mC cm−2 )

Ratio of −1 |Qpa × Qpc |

Potential limits

Cathodic (V)

P. Georgiou et al. / Electrochimica Acta 55 (2010) 1207–1216

0.059 2.2RT = nF n

(1)

where R is the gas constant, T the temperature in K, n the number of electrons, and F the Faraday constant. −1 −1 | or |Qpa × Qpc | should have unit value (ii) The ratio of |ipa × ipc [11,34]. Concerning the electrochemical treatment results presented in Table 2 for the materials (samples) of Fig. 2a, the corresponding values of Ep are higher than 0.059/n for n = 1, or they are much lower −1 for multielectron reactions. Furthermore, the ratio of |ipa × ipc | is close to unity for the first to the tenth cycle and it increases from −1 the tenth to the twentieth cycle. Similarly, the ratio of |Qpa × Qpc | is lower than unity for the first and second cycles and it increases from the fifth to the twentieth cycle. Therefore, the redox reactions cannot be characterized as fully reversible. Fig. 2b shows the cyclic voltammograms of the twentieth cycle of the pyrolyzed carbon fibres treated in the wide, W region, and the peak parameters are listed in Table 2. During the forward sweep, from −3.0 to +3.0 V, there is a broad anodic oxidation peak which becomes broader and is shifted to lower potential with increasing electrolyte concentration. This is due to more intense oxidation occurring on the carbon fibres treated in more concentrated H2 SO4 solution. During the electrochemical treatment of carbon fibres in 50% (w/w) H2 SO4 , a yellowish ‘inking’ of the solution after the third cycle was observed, which became more intense, changing to light brown, and then to dark brown with increasing number of cycles. This phenomenon indicates over-oxidation of the carbon fibres, especially for those treated for 20 cycles, leading to decomposition products and partially degraded structures of the carbon fibres. Furthermore, the broad anodic peak, previously described for the three concentrations of H2 SO4 is observed between +0.67 and +1.75 V (see Table 2 and Fig. 2b) and is attributed to the anodic peaks a2 , a3 and a4 seen in Fig. 1. Above the anodic p.l., oxygen evolution occurs as the main product due to electrolyte decomposition by electrolysis, and additionally, CO and CO2 are formed as byproducts due to the decomposition of oxygenous groups on carbon fibres caused by their over-oxidation [48]. The release of CO2 inside the fibre pores leads to the formation of carbonate ions which might be intercalated into graphitic planes at the surface of carbon fibre [56]. The anodic p.l. shifts to more positive potential with increasing electrolyte concentration. During the reverse sweep, from +3.0 to −3.0 V, a broad cathodic peak is observed at negative potentials, which is shifted to less negative potentials with increasing electrolyte concentration. This peak is attributed to the reduction of mainly ‘surface oxides’ and ‘defect oxides’. This cathodic peak is observed from −0.61 to −1.77 V (see Table 2 and Fig. 2b) for the three electrolyte concentrations, and is attributed to the cathodic peak c2 seen in Fig. 1. Below the cathodic p.l., hydrogen evolution occurs due to electrolyte decomposition by electrolysis. The cathodic p.l. shifts to more negative potentials with increasing electrolyte concentration. Concerning the electrochemical treatment results presented in Table 2 for the materials (samples) of Fig. 2b, the ipa and |ipc | decrease with increasing electrolyte concentration. According to −1 −1 the values of Ep and the ratios of |ipa × ipc | and |Qpa × Qpc | for each curve, it is concluded that the redox reactions are irreversible, i.e. the associated redox processes are under kinetic control and the oxygen containing groups formed during the anodic treatment

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Table 3 Surface elemental concentrations (at.%) determined by XPS. Sample

C 1s (∼284.40 eV)

O 1s (∼531.60 eV)

N 1s (∼ 400.00 eV)

S 2p (∼ 168.00 eV)

Auger O (KLL) (∼ 975 eV)

Notes: detection in traces of: Ca 2p (∼ 347.20 eV)

P P-C20-N-20c P-C5-W-20c P-C20-W-20c P-C50-W-20c

98.54 98.57 77.03 78.89 73.13

1.31 1.30 18.88 18.96 23.38

0.15 0.13 2.41 1.43 1.83

1.45 0.64 0.98

Weak Weak Weak

0.24 0.07 0.68

Fig. 3. Widescan spectrum of the carbon fibre P-C20-W-20c. Fig. 4. Ratio of the elemental concentrations: A = O 1s/C 1s, B = N 1s/C 1s and C = S 2p/C 1s based on Table 3.

of the carbon fibres are not fully reduced during their cathodic treatment. 3.2. X-ray photoelectron spectroscopy The surface elemental compositions determined from the widescan XPS spectra are presented in Table 3, and a typical widescan spectrum is shown in Fig. 3. The concentration of oxygen is low for the untreated carbon fibres, P, and the carbon fibres treated in the N region, P-C20-N-20c. The oxygen concentrations for the carbon fibres treated in the W region are high. For the latter, the oxygen concentration of the carbon fibres increases slightly with increasing electrolyte concentration from 5% to 20% and 50% (w/w). The nitrogen concentration on the carbon fibres is low. The carbon fibres treated in the W region contain sulfur and traces of calcium due to contamination [57], unlike those of the N region and the untreated carbon fibres. Therefore, we can classify the carbon fibres into two categories. The first category includes electrochemically untreated carbon fibres, sample P, and carbon fibres electrochemically treated under mild conditions, N region, and includes sample P-C20-N-20c, which display a high concentration of carbon and very low concentration of oxygen and nitrogen with no sulfur. The second category includes electrochemically treated carbon fibres under intense conditions, W region, and includes samples P-C5-W-20c, P-C20-W-20c, and P-C50-W20c. These fibres display intermediate carbon concentrations, high oxygen concentrations, and low concentrations of nitrogen and sulfur. In general, the presence of oxygen indicates that oxidation of the graphite has occurred, the presence of nitrogen is due to residues of pyridine like structures formed during the cyclization of PAN fibres [58] and the presence of sulfur indicates the presence of sulfur containing groups formed during the electrochemical treatment by cyclic voltammetry. The presence of sulfur as intercalated anion (HSO4 − ) in the carbon fibres treated in the W region should be excluded because of the low concentration of sulfur.

The surface elemental concentrations of oxygen, nitrogen and sulfur relative to that of carbon are presented in Fig. 4. The indices of oxygen (ratio A), nitrogen (ratio B) are very low for the carbon fibres of the first category, whilst, for the second category, the index of oxygen is high and the indices of nitrogen and sulfur (ratio C) are low.

Fig. 5. XPS C 1s high resolution spectrum of the carbon fibre P-C20-W-20c.

Table 4a Peak fit parameters for the C 1s photoelectron region. Sample

Peak 2 (*1 ), ␤-carbon (attached to oxidized carbon atoms)

Refs. [2,9,25–29]

Refs. [25,26,29]

Peak 3, C present in phenolic/alcohol or ether groups C–OH/C–O–C, pyridinic/quaternary C N, C–N, C6 H5 NH2 , C6 H5 NO2 , CNH2 , C≡N, C–S (after acid treatment) Refs. [1,2,14,25–29,59–64]

Peak 4 (*2 ), HBS - hydrogen bridged structure

Peak 5, C present in carbonyl or quinone groups C O, pyrrolic C–NH, pyridonic groups C–O–SO3 H

Refs. [1,25–27,58]

Peak 6, C present in carboxyl COOH or ester COOR groups, carbonate ions CO3 2−

Refs. [1,2,14,25–28,59,60,62] Refs. [2,14,25–29,60]

Peak 7, CO, CO2 adsorbed, bicarbonate ions CO3 H− , ␲–␲* transitions

Peak 8, surface plasmon interactions (no bonds)

Refs. [2,14,25–29,60]

Refs. [27,61]

Binding energy (eV)

FWHM (eV)

Curve fit Binding energy conc. (eV) (at.%)

FWHM (eV)

Curve fit Binding energy conc. (eV) (at.%)

FWHM (eV)

Curve fit Binding energy conc. (eV) (at.%)

FWHM (eV)

Curve fit Binding energy conc. (eV) (at.%)

FWHM (eV)

Curve fit Binding energy conc. (eV) (at.%)

FWHM (eV)

Curve fit Binding energy conc. (eV) (at.%)

FWHM (eV)

Curve fit Binding energy conc. (eV) (at.%)

FWHM (eV)

Curve fit conc. (at.%)

284.60 284.60 284.60 284.60 284.60

0.90 0.98 1.03 1.09 1.17

56.46 57.17 45.91 44.73 38.61

0.90 0.98 1.03 1.09 1.17

3.74 4.44 3.47 3.16 2.12

1.48 1.50 1.30 1.30 1.43

13.80 12.22 12.09 14.36 18.65

1.58 1.75 1.75 1.75 1.70

7.59 8.87 6.93 6.23 4.31

1.73 1.50 1.64 1.50 1.60

3.84 3.25 4.70 3.39 4.97

1.71 1.68 1.40 1.73 1.67

3.94 3.94 1.85 4.73 3.29

1.90 1.90 1.60 1.63 1.90

6.01 6.01 1.08 1.66 1.17

1.90 1.90 1.60 1.50 1.50

3.15 2.56 0.77 0.63 0.00

285.45 285.45 285.45 285.45 285.45

285.65 285.62 286.10 286.12 286.14

286.70 286.70 286.70 286.70 286.70

287.93 287.90 287.95 287.74 287.87

289.15 289.12 289.09 288.76 288.87

290.73 290.67 290.46 290.43 290.72

292.63 292.59 291.30 291.81 296.03

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P P-C20-N-20c P-C5-W-20c P-C20-W-20c P-C50-W-20c

Peak 1, graphitic carbon C C, hydrocarbon C–C

.

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Table 4b Peak fit parameters for the O 1s photoelectron region and single peak for N 1s and S 2p photoelectron spectra. Sample

P P-C20-N-20c P-C5-W-20c P-C20-W-20c P-C50-W-20c

O 1s

N 1s

S 2p

Peak 1, oxygen present in carbonyl/lactone C O groups (no quinone groups ∼530.5 eV), HBS structure, O C–OH carboxyl groups, oxygen in carbonates, S O Refs. [2,26–28,64,65]

Peak 2, oxygen present in alcohol/phenol C–OH or ether C–O–C groups, O C–OH carboxyl groups or ester groups, S–O

Peak 3, C N–O, chemisorbed oxygen and adsorbed water

Peak, pyrrolic N (N-5), pyridonic N (N-6)

Peak, SO3 − sulfonate –SO3 H (=C–SO3 H, N–SO3 H), sulfate –OSO3 H

Refs. [2,26–28,64,65]

Refs. [27,28,60,66]

Refs. [14,60–63]

Refs. [59,64,65]

Binding energy (eV)

FWHM (eV)

Curve fit conc. (at.%)

Binding energy (eV)

FWHM (eV)

Curve fit conc. (at.%)

Binding energy (eV)

FWHM (eV)

Curve fit conc. (at.%)

Binding energy (eV)

Binding energy (eV)

531.60 531.09 531.32 531.24 531.28

2.02 2.20 1.93 1.96 1.80

0.10 0.08 6.84 3.89 5.72

532.43 532.42 532.67 532.67 532.68

2.02 2.20 1.93 1.96 1.80

1.14 1.07 11.54 14.27 16.82

534.30 535.30 535.20 535.19 534.64

2.02 2.20 1.93 1.96 1.80

0.07 0.15 0.50 0.80 0.84

399.8 399.8 399.8 399.8 399.8

167.9 167.9 167.9 167.9 167.9

In addition to the widescan XPS spectra, high energy resolution spectra were obtained from the C 1s region. Peak fitting was performed on these spectra, and the results for sample P-C20-W-20c are shown in Fig. 5. Eight peaks, with binding energies based on literature values, were required and the values for all samples are compiled in Table 4a. The peak fit data display the relative areas of the fitted components. To obtain the absolute atomic concentrations of the curve fitted components we simply multiply the elemental concentration obtained from the widescans by the curve fitted fractional intensity. The C 1s peak widths for sample P-C20W-20c were constrained to the C–C peak width, which was allowed to change between 1.0 to 1.5 eV. The carbon fibres P and P-C20N-20c exhibit narrow C 1s peak shape normally associated with untreated graphite fibres [26], with FWHM of the main graphitic peak of 0.90 and 0.98 eV respectively. The least graphitic nature is observed for carbon fibres where the FWHM of the main graphitic peak is greater than 1.0 eV, which occurs for samples P-C5-W-20c, P-C20-W-20c and P-C50-W-20c. This suggests that disordering of the carbon lattice occurs upon oxidation [25,27]. The values of the curve fit concentrations for Peak 1, graphitic carbon, are greater than 50 at.% for samples P and P-C20-N-20c, and are significantly lower for all other samples. As has been mentioned previously, samples P and P-C20-N-20c belong to the first category and the samples P-C5-W-20c, P-C20-W-20c and P-C50W-20c belong to the second category. Consequently, the carbon fibres of the second category contain more functional groups than those of the first category, which is in agreement with the results of Fig. 4 and Table 3. The results described previously were based on analysis of the C 1s spectra. Fig. 6 shows the O 1s high energy resolution spectrum of sample P-C20-W-20c. The parameters used in peak fitting of the O 1s photoelectron region are listed in Table 4b. The peak widths were constrained to the O–C peak width, which was allowed to change between 1.8 and 2.0 eV.

Fig. 6. XPS O 1s high resolution spectrum of the carbon fibre P-C20-W-20c.

The atomic concentrations of the O 1s components are not stoichiometric with those of the C 1s region due to the different sampling depths of the C 1s and O 1s photoelectrons and the strong possibility of in-depth inhomogeneity. The functional groups that are detected in the O 1s region are located on the outer surface since these photoelectrons have a lower kinetic energy [25,27]. The classification of the carbon fibres into the two categories, previously described, is also supported by the results from the O 1s region. The values of the curve fit concentrations, listed in Table 4b, are very low for the carbon fibres of the first category, peaks 1–3, opposed to those of the second category which are significantly higher. The second category therefore contains more oxygen containing functional groups. Peak fitting to the high energy resolution S 2p photoelectron spectra required only a single spin orbit split doublet with a S 2p3/2 binding energy of 167.9 eV, indicating the presence of sulfate. The high energy resolution N 1s photoelectron spectra contain only a single peak at 399.8 eV binding energy. Their surface elemental con-

Table 5 Indices of carbon – carbon bonds and of carbon bonded to oxygen. Sample

Index A (C–C) Peak 1 + peak 2 + peak 7 C–C, C C + ␤-carbon + ␲–␲* transitions (at.%)

Index B (C–O) Peak 3 + peak 4 + peak 5 + peak 6 Total oxides (at.%)

Index C Index A Index B

P P-C20-N-20c P-C5-W-20c P-C20-W-20c P-C50-W-20c

66.21 67.62 50.46 49.55 41.90

29.17 28.28 25.57 28.71 31.22

2.27 2.39 1.97 1.73 1.34

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Fig. 7. Adsorption of methylene blue from aqueous solution on untreated and electrochemically treated carbon fibres (see Table 1). X: adsorbed amount of methylene blue, X0 : initial amount of methylene blue in the solution before the adsorption. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

centration is given in Table 3. Details of peak positions and their assignments with respect to the literature are included in Table 4b. By using the relative intensities determined by peak fitting to the high energy resolution C 1s spectra, a truer representation of the surface chemistry is obtained since they originate from the same sampling depth. Indices have been calculated to reflect the degree of surface oxidation, and are presented in Table 5. Index A is the sum of peak intensities of carbon bonded to carbon, including ␤-carbon and ␲–␲* transitions, as these transitions are losses from C–C bonds. Index B is the sum of peak intensities of carbon bonded to oxygen. Index C is simply the ratio of indices A and B. According to the values of INDEX C, these are above to the value 2 for the carbon fibres of the first category and lower than the value 2 for the carbon fibres of the second category. 3.3. Dye adsorption Fig. 7 shows the time dependence of discolouration of methylene blue from aqueous solution by the carbon fibres. The untreated carbon fibres, sample P, and those treated in the N region, sample PC20-N-20c, show practically no discolouration of methylene blue solution, indicated by equilibrium adsorption of only 7% and 6% respectively, reached after 75 min. The carbon fibres treated in the W region, samples P-C5-W-20c, P-C20-W-20c and P-C50-W-20c, showed complete discolouration of the methylene blue solution indicated by 97%, 93% and 98% dye adsorption respectively, with equilibrium attained after 3, 3.5 and 1.5 h respectively. Fig. 8 shows the time dependence of discolouration of alizarin yellow from aqueous solution by the carbon fibres. The untreated carbon fibres, sample P, show 15.6% adsorption of alizarin yellow at equilibrium, reached after 75 min. The carbon fibres treated in the N region, sample P-C20-N-20c, show slightly increased adsorption of 20.8% at equilibrium reached after 1.5 h. The carbon fibres electrochemically treated in the W region, samples P-C5-W20c, P-C20-W-20c and P-C50-W-20c, showed 100%, 100% and 64% adsorption at equilibrium reached after 48, 48 and 2.5 h respectively. The lower dye adsorption of sample P-C50-W-20c compared to samples P-C5-W-20c and P-C20-W-20c indicates over-oxidation of the carbon fibres. This is in agreement with the ‘inking’ of the electrolyte solution observed during the electrochemical treatment of carbon fibres in electrolyte of 50% (w/w) concentration. The mechanism of uptake of each dye is different [67–69]. Methylene blue (C16 H18 ClN3 S), is a cationic or basic dye; the organic cations, which are the functional part of the dye, are

Fig. 8. Adsorption of alizarin yellow from aqueous solution on untreated and electrochemically treated carbon fibres (see Table 1). X: adsorbed amount of alizarin yellow, X0 : initial amount of alizarin yellow in the solution before the adsorption. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)

attracted to the anionic parts eg. in acrylic fibres, and they form electrovalent bonds. The organic cations contain S+ and N+ (CH3 )2 , which are both electron donors. Alizarin yellow (C13 H9 N3 O5 ), is an acidic dye or anionic monoazo dye, which contains –NO2 , –N N–, and –COOH that are electron acceptors, and –OH that is an electron donor. The adsorption of methylene blue and alizarin yellow dyes on carbon fibres can be interpreted by “electron donor–acceptor interactions” between the characteristic groups present on carbon fibre surface and those of the dyes [18,70]. Methylene blue adsorption is favoured by the presence of electron acceptor groups on the carbon fibre surface, whilst alizarin yellow adsorption is favoured by the presence of electron donor groups on the carbon fibre surface. The second category of carbon fibres adsorb both methylene blue and alizarin yellow very effectively, although uptake of alizarin yellow on sample P-C50-W-20c is lower, indicating a high concentration of both electron acceptor groups, such as C O, COOR and COOH, and of electron donor groups, such as C–OH, C–O–C. The carbon fibres of the first category exhibit very low adsorption of both methylene blue and alizarin yellow indicating low concentrations of both electron acceptor and electron donor groups at the surface. This indicates that electrochemical modification of carbon fibre surfaces may be achieved using conditions which were applied to the second category. 4. Conclusions Commercial carbon fibres were pyrolyzed up to 1000 ◦ C and were then electrochemically treated by cyclic voltammetry in aqueous electrolyte solutions of H2 SO4 , in two potential sweep ranges: a narrow region, N, and a wide region, W, avoiding and including water decomposition, respectively. Characterisation of the electrochemically treated carbon fibres by XPS and dye adsorption allows their classification into two categories. The first category includes the untreated fibres, sample P, and fibres electrochemically treated carbon under mild conditions, N region, sample P-C20-N-20c. The second category includes fibres electrochemically treated under intense conditions, W region, samples P-C5-W-20c, P-C20-W-20c and P-C50-W-20c. Carbon fibres in

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the first category show low surface concentrations of oxygen and nitrogen, whilst those in the second, show high concentrations of oxygen and nitrogen, and low concentrations of sulfur. Peak fitting to the high energy resolution photoelectron spectra allows the ratio of the relative intensity of oxygen bonded to carbon to carbon bonded to carbon to be determined. For fibres in the first category this ratio is greater than 2, whilst for those in the second category it is less than 2, indicating greater oxidation of the fibre surface in the first category and lower in the second. This is corroborated by adsorption of dye by the carbon fibres. The carbon fibres of the second category adsorb methylene blue dye very effectively, indicating the presence of electron acceptor groups, and adsorb alizarin yellow dye effectively, indicating the presence of electron donor groups in lower concentration. The electrochemical conditions used for the treatment of the carbon fibres in the second category are therefore suitable for modification of the fibre surface; however over-oxidation shown by sample P-C50-W-20c leads to partially degraded structures. XPS is a useful tool in determining the presence and relative intensity of functional groups on carbon fibre surfaces, enabling their surface chemistry to be elucidated. However, absolute quantification is complicated by in-depth inhomogeneity and changes in the sampling depth due to the geometry of the fibres. The adsorption of dye on carbon fibre surfaces allows an indirect estimation of the extent of oxidation, without providing details of their surface chemistry. Acknowledgements The author P. Georgiou wants to thank the Greek State Scholarship’s Foundation (IKY) for its PhD scholarship. The authors P. Georgiou and J. Simitsis want to thank Dr. I. Liakos (EC – JRC – IHCP/ISPRA) for his help in the initial stage of this work concerning the XPS method. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]

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